The present invention relates to a method for applying thermal energy to a recording medium, using a thermal head having energisable heating elements which are individually addressable. More specifically the invention concerns a method for calibrating a continuous tone thermal printer. In particular, the recording medium is a thermographic material, and the method for thermal printing relates to thermography.
Thermal imaging or thermography is a recording process wherein images are generated by the use of imagewise modulated thermal energy. Thermography is concerned with materials which are not photosensitive, but are sensitive to heat or thermosensitive and wherein imagewise applied heat is sufficient to bring about a visible change in a thermosensitive imaging material, by a chemical or a physical process which changes the optical density.
Most of the direct thermographic recording materials are of the chemical type. On heating to a certain conversion temperature, an irreversible chemical reaction takes place and a coloured image is produced.
In direct thermal printing, the heating of the thermographic recording material may be originating from image signals which are converted to electric pulses and then through a driver circuit selectively transferred to a thermal print head. The thermal print head consists of microscopic heat resistor elements, which convert the electrical energy into heat via the Joule effect. The electric pulses thus converted into thermal signals manifest themselves as heat transferred to the surface of the thermographic material, e.g. paper, wherein the chemical reaction resulting in colour development takes place. This principle is described in xe2x80x9cHandbook of Imaging Materialsxe2x80x9d (edited by Arthur S. Diamondxe2x80x94Diamond Research Corporationxe2x80x94Ventura, Calif., printed by Marcel Dekker, Inc. 270 Madison Avenue, New York, ed. 1991, p. 498-499).
A particular interesting direct thermal imaging element uses an organic silver salt in combination with a reducing agent. An image can be obtained with such a material because under influence of heat the silver salt is developed to metallic silver.
A thermal printer varies the printing energy to control the density of the thermal print. The objective is to print predictable densities with minimum increments to produce a nearly continuous grey scale over the desired density range. Typically, control is a two stage process.
A traditional technique for calibrating a thermal printer is as follows.
First, a first calibration page is printed with a limit setting to produce the desired maximum density and a full range of print settings. The next step is to determine whether this is the desired limit setting by visually inspecting the printed page. The normal objective is to find the minimum exposure required to print the full range of desired densities. The lower the limit setting, the more nearly continuous the grey scale in the printed film. The process of printing and adjusting the maximum limit setting is repeated until a desired limit setting is determined.
Next, a second calibration page is printed with the limit system setting selected and with a subset of print system settings which cover the full range of print settings. The resulting densities of the printed page are then measured and a print setting to density table created for the full range of print settings. An output lookup table that can be used to set exposure to produce the desired density for any digital image value is created using the print setting to density table. Thereafter the thermal printer prints pages with this output lookup table to produce the desired densities while the same maximum exposure is appropriate.
However, if maximum exposure is changed the calibration process must be repeated.
A problem which arises with this calibration technique is that calibration data is specific to a particular limit control setting. If that setting needs to be changed the entire process of successive prints to find the desired limit control setting for maximum density and calibration must be repeated. Also, if different users want different maximum densities each requires separate calibration. Such repeated calibrations is inefficient, costly and non-productive.
It is an aspect of the present invention to provide an improved calibration method for recording an image on a thermal imaging element by means of a thermal head having energisable heating elements.
It is a further aspect of the invention that the calibration method has the ability to produce a single calibration page and to derive from that single page sufficient information to produce calibrated prints over a wide range of densities.
Other aspects and advantages of the present invention will become clear from the description and the drawings.
The above mentioned aspects are realised by a calibration method having the characteristics defined in the independent claims. Specific features for preferred embodiments of the invention are set out in the dependent claims.
Further advantages and embodiments of the present invention will become apparent from the following description.